The invention relates to an on-board electrical system isolating circuit for a DC/DC converter, in particular for a push-pull rectifier with synchronous rectification, and to a method for isolating an on-board electrical system from a DC/DC converter.
Synchronous rectifier circuits are usually used in potential-isolating DC/DC converters, for example for supplying a low-voltage on-board electrical system of a vehicle. The power semiconductor switches, for example MOSFETs, used for this purpose have a lower voltage loss than diodes precisely in the case of relatively high DC currents, as a result of which the efficiency of the rectifier can be increased.
During the operation of the synchronous rectifier circuits, overvoltages may occur at the power semiconductor switches and may overload and hence permanently damage the power semiconductor switches. Therefore, countermeasures such as snubber circuits are usually used, which are intended to avoid overvoltages at the power semiconductor switches.
It may nevertheless happen that a power semiconductor switch in the synchronous rectifier circuit is damaged and as a consequence switches permanently in the on state. In such a case, an undesired reverse current from the secondary-side on-board electrical system into the synchronous rectifier circuit occurs, whereby electrical energy stores of the on-board electrical system may be discharged. If the on-board electrical system can no longer be supplied with enough energy, specific vehicle components may also fail in their function. For safety reasons it is therefore desirable to prevent such reverse currents in the case of a fault in the synchronous rectifier circuit.
The document DE 41 38 943 C1 for example discloses a two-voltage on-board electrical system, with a DC/DC converter arranged between the two partial on-board electrical systems, said DC/DC converter being part of a charging/isolating module which, depending on signals fed, for example depending on measured currents, interrupts the connection between the two partial on-board electrical systems and thus prevents perturbing reactions from one partial on-board electrical system into the other in the case of a fault.